The present invention relates to a coated optical fiber or, more particularly, to an optical fiber coated with a layer of a cured organopolysiloxane curable by irradiation with actinic rays and having low water absorption as well as small temperature dependency of the Young's modulus as well as a method for the preparation of such a coated optical fiber.
As is known, various materials are used for optical fibers in service of the optical communication including fused quartz glass, multi-component glass and plastics. In view of the lightweight, low transmission loss, absence of induction, heat resistance and weatherability as well as large transmission capacity, however, most of the optical fibers in practical services are made of fused quartz glass. It is usual that quartz glass-made optical fibers are provided with a coating layer of a certain protecting material since quartz glass fibers are usually very small in diameter and subject to changes of the performance in the lapse of time.
Various materials have been proposed for the coating layer on quartz glass-made optical fibers including so-called silicones and organic polymers. For example, a typical silicone material for optical fiber coating is an organopolysiloxane composition comprising an organopolysiloxane having vinyl groups bonded to the silicon atoms and an organohydrogenpolysiloxane having hydrogen atoms directly bonded to the silicon atoms, of which a cured coating layer is formed by the addition reaction or so-called hydrosilation between the silicon-bonded vinyl groups and silicon-bonded hydrogen atoms in the presence of a platinum catalyst. Another curable silicone material for optical fiber coating is an organopolysiloxane composition comprising a vinyl-containing organopolysiloxane and a mercaptoalkyl-containing organopolysiloxane to effect an addition reaction between the vinyl and mercapto groups. Organic polymers include those by use of a polyether urethane acrylate or polybutadiene urethane acrylate.
These prior art coating materials for optical fibers have their respective problems and disadvantages. For example, the organopolysiloxane compositions by the hydrosilation reaction have a problem that the organohydrogenpolysiloxane is susceptible to the reaction with atmospheric moisture to evolve hydrogen gas which causes increase in the transmission loss through the optical fiber coated therewith. The organopolysiloxane compositions by the mercapto-vinyl addition reaction are sometimes unacceptable due to the very unpleasant odor ascribable to the mercapto groups emitted in the course of the curing reaction. The coating materials based on a polyether urethane acrylate have disadvantages of large water absorption and unduly high rigidity at low temperatures to increase the transmission loss of the coated optical fibers while those based on a polybutadiene urethane acrylate are disadvantageous in respect of the low curability. Accordingly, it is eagerly desired to develop a novel optical fiber coated with a coating material free from the above described problems and disadvantages in the prior art.